Air conditioning systems in vehicles must normally be able to function in greatly varying conditions, for instance regarding ambient temperature and atmospheric humidity. Also, its function depends on the quantity of refrigerant in the closed system.
The most common cause of malfunctions in air conditioning systems is a too high or too low refrigerant quantity. A small leakage in an air conditioning system can be very difficult to discover. If the compressor, over a very long period of time, is forced to function with a too low refrigerant quantity, it might break down.
Even a slight shortage of refrigerant can lead to the following negative consequences:
low undercooling after the condensor, PA1 high overheating after the evaporator, PA1 the evaporation pressure is lower than normal, and PA1 the output temperature of the compressor is higher than normal. PA1 high undercooling after the condensor, PA1 the condensing pressure is higher than normal, PA1 the output temperature of the compressor is lower than normal, PA1 the evaporation pressure is higher than normal, and PA1 the overheating after the evaporator is low.
An excess of refrigerant might, on the other hand, lead to the following negative consequences:
In practice, a shortage as well as an excess of the refrigerant will force the compressor to work more often and during longer time periods than what otherwise would have been necessary.
Up to now, no suitable devices have existed for measuring the refrigerant quantity in a closed system with capillary tubes as a shut-off means, and with a liquid-separating accumulator after the evaporator. For this reason, service and maintenance of cars equipped with air conditioning systems usually mean that the liquid must be evacuated from the system when measuring the quantity of the refrigerant. This evacuation is time consuming, since the system comprises rather long fluid lines with narrow passages. Draining and filling of the refrigerant also lead to risks of discharging substances which are harmful to the environment.
U.S. Pat. No. 4,757,693 describes an air conditioning system with an electronic circuit for detection of malfunctions, for example due to an inaccurate refrigerant quantity. This air conditioning system is however equipped with a tank for the refrigerant between the condensor and the expansion valve. U.S. 4,308,725 describes a detector for indicating the level of the refrigerant in such a storage tank.